HOW TO CHECK MOISTURE IN CONCRETE SURFACE
Concrete slabs appear to be solid and hard, but in reality, concrete is moist stuff . A concrete surface may look dry, but the slab can still contain sufficient moisture to cause problems when covered with flooring or with paints .
Concrete moisture is understood to mean the total water used in the concrete batch, plus curing water, minus the water bound in hardened cement due to hydration. The amount of concrete moisture can be considerable. In practical terms, several kg of water must evaporate from every square foot of concrete for the slab to be considered adequately dry for floor finishes.
Best and cheap moisture meter for woodA method of testing the moisture condition of a concrete slab is to measure the relative humidity of the air in the concrete pore system. This is done by placing a relative humidity probe into a hole drilled in the concrete. The relative humidity achieved within a concrete slab depends on a combination of factors including the initial water-to-cement ratio, drying history, pore structure, and concentration of soluble ions in the pore water solution.
Note :- This method is very old so in these days we use moisture meter to check the moisture in concrete , plaster , wood etc.
Moisture meters can provide a quick and reliable “wet/dry” indication for concrete. Simply test a bit of concrete you know to be dry with the meter and make a note of the reading given. Then, use the meter to test the rest of the concrete
If a reading is significantly higher than the sample you tested, then that section of concrete is probably wet and needs more exhaustive testing per the ASTM F-2170 standard.